This invention relates to detergent compositions and, more particularly, to phosphate-free detergent compositions employing alkali metal carbonate and citrate builder salts.
Current detergent formulations contain large amounts of phosphate salts, primarily in the form of polyphosphates and orthophosphates. These phosphates have been found to be highly effective sequestering agents, but they are not without disadvantages. Phosphates and phosphate-containing detergent formulations have recently received considerable attention as prime suspects in water pollution. Phosphates are principally alleged to be causative in accelerated eutrophication of the nation's waters, and there has recently been an increasing demand for effective detergent compositions which are low in phosphates or, preferably free of phosphates.
Hard water contains an excess of Ca.sup.+.sup.2 ions which interfere with the cleaning process. Phosphate ions are the most effective sequestrant currently in use in detergent formulations, but much effort is being directed toward reducing and eventually completely replacing phosphate salts in detergents. As the availability of other sequestrants is presently limited, it is no longer possible to have a high concentration of sequestrant in detergent formulations, and another method must be found for eliminating Ca.sup.+.sup.2 ions from wash water.
Recently, attention has been directed to detergent formulations containing sodium carbonate to ameliorate the problem of washing in hard water. Where the detergent formulation contains mainly a surfactant and sodium carbonate, it is preferable to have the sodium carbonate acting as a water softener, and precipitation of the calcium ions is necessary. Where sodium carbonate is used primarily as a water softener in the laundry process, it is preferred to tie up Ca.sup.+.sup.2 ions as soon as possible by interaction with the carbonate ions supplied by the sodium carbonate. Additionally, it is desirable to have good suspension of the resulting calcium carbonate precipitate in the wash water, in order to minimize adsorption of calcium carbonate onto fabric substrates and thus decrease fabric "boardiness".
At the present time the effect of calcium carbonate particles on the cleaning process is not well defined, but one can reasonably speculate that it is better for soil removal to maintain calcium carbonate particles in the bulk solution rather than to let them settle onto the dirt-fabric substrate.
The possible approaches for minimizing calcium carbonate adsorption are as follows:
(a) retard calcium carbonate precipitation until after the wash cycle PA0 (b) delay calcium carbonate precipitation until the latter stages of the wash cycle PA0 (c) promote calcium carbonate suspension PA0 (d) enhance the efficiency of the rinse cycle PA0 (e) redesign the washing machine PA0 (f) use additives during the rinse cycle which can either redissolve calcium carbonate or remove calcium carbonate from fabric substrates PA0 (a) Ca.sup.+.sup.2 + Sequestering anion.revreaction. Ca-Sequestrant Complex PA0 (b) Ca.sup.+.sup.2 + Precipitating anion.revreaction. Ca-Precipitation PA0 (a) Ca.sup.+.sup.2 + Sequestering anion.revreaction. Ca-Sequestrant Complex PA0 (b) Ca.sup.+.sup.2 + Precipitating anion.revreaction. Ca-Precipitation PA0 a. As long as reaction occurs between the Ca.sup.+.sup.2 ions and the sodium citrate, sodium carbonate will have little effect on this interaction during the washing cycle. PA0 b. As one minute is required to induce calcium carbonate precipitation, it is not necessary to predissolve sodium citrate in order to prevent precipitation of calcium carbonate. PA0 c. During the precipitation of calcium carbonate, the addition of sodium citrate will have some effect, e.g., by decreasing the amount of calcium carbonate precipitation. PA0 d. If calcium precipitation is complete, sodium citrate will have little effect on it, as sodium citrate is believed to be a weak calcium sequestrant.
The first two approaches are preferred because they best retain the original cleaning performance of the detergent. It is important to minimize calcium carbonate adsorption from sodium carbonate-containing detergent formulations.
It is generally agreed that the process of precipitation proceeds by two stages, beginning with nucleation onto impurity particles or seeds. Subsequently, the nuclei grow into visible crystallites. The crystallites may form a stable suspension or they may coagulate. When they become larger they tend to sediment.
The possible variables which can effect the interaction between the Ca.sup.+.sup.2 ions and the carbonate ions are classified into two types, controllable and uncontrollable, based on the actual laundry conditions. Controllable variables include concentration of carbonate ions, surfactants and other detergent ingredients, the specific properties of the detergent ingredients, ionic strength, pH of the solution, and other crystallization variables such as inhibitors or promoters of precipitation. Uncontrollable variables include hardness of water, temperature, substrates (fabric, dirt and, impurities), and some other crystallization variables. There are some correlations among these variables, such as a minimum required hardness for a specific detergent formulation in order to have precipitation occur.
As the amount of sequestrant present in detergent formulations is gradually being reduced, it is of interest to determine the amount of a sequestrant needed for calcium control only in the laundry process to aid in formulating an effective detergent composition.
If a detergent system contains both sequestering and precipitating agents, the two anions will compete for the calcium ions as follows:
Here, (a) process is generally much more rapid than (b) process. If the sequestrant is strong, (b) process will have little effect on (a) process, regardless of which one proceeds first. If the sequestrant is weak, there will be interference between the two processes. The final equilibrium of each process will depend on the order of occurrence and the amounts of agents used.